Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory, including volatile and non-volatile memory. Volatile memory requires power to maintain its data, and examples of volatile memory include random-access memory (RAM), dynamic random-access memory (DRAM), and synchronous dynamic random-access memory (SDRAM), among others. Non-volatile memory can retain stored data when not powered, and examples of non-volatile memory include flash memory, read-only memory (ROM), electrically erasable programmable ROM (EEPROM), static RAM (SRAM), erasable programmable ROM (EPROM), resistance variable memory, such as phase-change random-access memory (PCRAM), resistive random-access memory (RRAM), magnetoresistive random-access memory (MRAM), and three-dimensional (3D) XPoint™ memory, among others.
Flash memory is utilized as non-volatile memory for a wide range of electronic applications. Flash memory devices typically include one or more groups of one-transistor, floating gate or charge trap memory cells that allow for high memory densities, high reliability, and low power consumption. Two common types of flash memory array architectures include NAND and NOR architectures, named after the logic form in which the basic memory cell configuration of each is arranged. The memory cells of the memory array are typically arranged in a matrix. In an example, the gates of each floating gate memory cell in a row of the array are coupled to an access line (e.g., a word line). In a NOR architecture, the drains of each memory cell in a column of the array are coupled to a data line (e.g., a bit line). In a NAND architecture, the memory cells in a string of the array are coupled together in series, source to drain, between a source line and a bit line.
Both NOR and NAND architecture semiconductor memory arrays are accessed through decoders that activate specific memory cells by selecting the word line coupled to their gates. In a NOR architecture semiconductor memory array, once activated, the selected memory cells place their data values on bit lines, causing different currents to flow depending on the state at which a particular cell is programmed. In a NAND architecture semiconductor memory array, a high bias voltage is applied to a drain-side select gate (SGD) line. Word lines coupled to the gates of the unselected memory cells of each group are driven at a specified pass voltage (e.g., Vpass) to operate the unselected memory cells of each group as pass transistors (e.g., to pass current in a manner that is unrestricted by their stored data values). Current then flows from the source line to the bit line through each series coupled group, restricted only by the selected memory cells of each group, placing current encoded data values of selected memory cells on the bit lines.
Each flash memory cell in a NOR or NAND architecture semiconductor memory array can be programmed individually or collectively to one or a number of programmed states. For example, a single-level cell (SLC) can represent one of two programmed states (e.g., 1 or 0), representing one bit of data. However, flash memory cells can also represent one of more than two programmed states, allowing the manufacture of higher density memories without increasing the number of memory cells, as each cell can represent more than one binary digit (e.g., more than one bit). Such cells can be referred to as multi-state memory cells, multi-digit cells, or multi-level cells (MLCs). In certain examples, MLC can refer to a memory cell that can store two bits of data per cell (e.g., one of four programmed states), a triple-level cell (TLC) can refer to a memory cell that can store three bits of data per cell (e.g., one of eight programmed states), and a quad-level cell (QLC) can store four bits of data per cell. Unless otherwise clearly indicated by express language or context, MLC is used herein in its broader context, to can refer to any memory cell that can store more than one bit of data per cell (i.e., that can represent more than two programmed states).
Traditional memory arrays are two-dimensional (2D) structures arranged on a surface of a semiconductor substrate. To increase memory capacity for a given area, and to decrease cost, the size of the individual memory cells has decreased. However, there is a technological limit to the reduction in size of the individual memory cells, and thus, to the memory density of 2D memory arrays. In response, three-dimensional (3D) memory structures, such as 3D NAND architecture semiconductor memory devices, are being developed to further increase memory density and lower memory cost.
Such 3D NAND devices often include strings of storage cells, coupled in series (e.g., drain to source), between one or more source-side select gates (SGSs) proximate a source, and one or more drain-side select gates (SGDs) proximate a bit line. In an example, the SGSs or the SGDs can include one or more field-effect transistors (FETs) or metal-oxide semiconductor (MOS) structure devices, etc. In some examples, the strings will extend vertically, through multiple vertically spaced tiers containing respective word lines. A semiconductor structure (e.g., a polysilicon structure) may extend adjacent a string of storage cells to form a channel for the storage cells of the string. In the example of a vertical string, the polysilicon structure may be in the form of a vertically extending pillar. In some examples, the string may be “folded,” and thus arranged relative to a U-shaped pillar. In other examples, multiple vertical structures may be stacked upon one another to form stacked arrays of storage cell strings.
Memory arrays or devices can be combined together to form a storage volume of a memory system, such as a solid-state drive (SSD), a Universal Flash Storage (UFS™) device, a MultiMediaCard (MMC) solid-state storage device, an embedded MMC device (eMMC™), etc. An SSD can be used as, among other things, the main storage device of a computer, having advantages over traditional hard drives with moving parts with respect to, for example, performance, size, weight, ruggedness, operating temperature range, and power consumption. For example, SSDs can have reduced seek time, latency, or other delay associated with magnetic disk drives (e.g., electromechanical, etc.). SSDs use non-volatile memory cells, such as flash memory cells, to obviate internal battery supply requirements, thus allowing the drive to be more versatile and compact.
An SSD can include a number of memory devices, including a number of dies or logical units (e.g., logical unit numbers or LUNs), and can include one or more processors or other controllers performing logic functions required to operate the memory devices or interface with external systems. Such SSDs may include one or more flash memory die, including a number of memory arrays and peripheral circuitry thereon. The flash memory arrays can include a number of blocks of memory cells organized into a number of physical pages. In many examples, the SSDs will also include DRAM or SRAM (or other forms of memory die or other memory structures). The SSD can receive commands from a host in association with memory operations, such as read or write operations, to transfer data (e.g., user data and associated integrity data, such as error data and address data, etc.) between the memory devices and the host, or erase operations to erase data from the memory devices.
An SSD has the potential for failure and data loss. A method for protecting data in an SSD has included adding levels of error correction code (ECC) to pages and then recovering data using the ECC. A protection scheme beyond ECC has included utilizing approaches at the device level with the use of multiple memory devices, providing redundant array of independent NAND (RAIN) protection. Versions of SSD data protection technology in these approaches have varied.
Some SSD controllers use parallelism in order to increase SSD performance and locate stored data across a set of flash devices, which may be realized as many relatively smaller flash devices to attain a large capacity associated with other storage devices. This parallelism in SSDs spreads spread across multiple of the flash devices of the set. This can be referred to striped data without parity. RAIN technology adds user data protection that extends beyond ECC, minimally impacts drive performance, and can optimize NAND management. With a high degree of parallelism already in place within the SSD by striping, adding a parity protection architecture adds another layer of protection.
RAIN is an umbrella term for data storage schemes that divide and/or replicate data among multiple pages of multiple memory devices, for instance, in order to help protect the data stored in the memory devices. The multiple memory devices in a RAIN array may appear to a user and an operating system of a computing machine as a single memory device. RAIN can include striping (e.g., splitting) data so that different portions of the data are stored on different pages of different memory devices. The portions of the different memory devices that store the split data are collectively referred to as a stripe. In an architecture for a particular RAIN design, a number of the pages of memory cells in a memory array can store a parity portion of a RAIN stripe. For instance, each respective one of the number of pages can include a parity portion of a different RAIN stripe. Since a RAIN stripe can be a combination of user data, other data, and parity data, the parity data can reduce the capacity of the SSD to store user data and can affect the performance of the SSD. RAIN can also include mirroring, which can include storing duplicate copies of data on more than one page of more than one device.
Though MLC NAND technology has enabled high-performance data storage to be widely available at affordable price points, MLC NAND has its limitations. For example, MLC devices are vulnerable to data loss in the event of an unexpected power loss. Frequently, source power to SSDs is shut down. Under normal circumstances, when a computer system is shut down or when it goes into a sleep mode or a hibernate mode, the host computer sends a command signal to the storage device to indicate that the power is going to turn off. Typically, the host system does not shut down until the command is sent and the drive acknowledges the command back to the host computer. The SSD completes any unfinished writes and saves any updated data addressing information before sending this acknowledge message.
An unexpected power loss, on the other hand, is a power loss that is not preceded by a shutdown notification. As a result, the drive cannot close out the final operations before loss of power, which can lead to data and addressing information being lost. Other common terms to describe this situation are “surprise power loss,” “dirty power loss,” and “asynchronous power loss (APL).”